Evan,
Is it the yellow to light-tan expanding polyurethane foam? I remember an old
Alpha-jet kit in 1/48 that used EPU foam for the main airframe components.
If so it forms a skin as it expands and cures, and can then be laminated over
with polyester or epoxy resins, it does not melt like polystyrene foam.
So the surface is pretty much sealed already, and is actually resistant to
most solvents, meaning it can be a bitch to work with when gluing part to part.
Give it a light scrubbing with a scotchbrite pad, to improve adhesion, and hit it with
the primer of your choice.
http://www.shopmaninc.com/foam.html